Directional couplers often are formed as waveguide, stripline, or microstrip directional couplers. Typical waveguide directional couplers are used primarily to sample power for measurements. For example, two waveguides could be located side-by-side, one above the other, parallel, or crossing each other. Holes can be drilled in a common wall to permit coupling between the waveguides.
A stripline or microstrip coupler, on the other hand, usually has a main transmission line in close proximity to a secondary transmission line. In some designs, quarter wavelength coupling sections are added to either side of a center section to increase bandwidth and reduce ripple. These quarter wavelength sections are less tightly coupled than the center section, and are equally disposed about the center section. For some microstrip applications, the velocity of propagation is different for even and odd modes, and to compensate for this difference, sometimes a capacitor is added to increase the localized capacitance and improve the directivity of the coupler. In these types of systems, when two coupler lines are in close proximity and the phase of energy is the same, an even mode symmetry of fields is accomplished. When the fields are 180 degrees out of phase, however, there is an odd mode symmetry.
High-directivity couplers are desirable for a wide variety of applications, including terrestrial transceivers or subsystems, test equipment and laboratory components. In the case of terrestrial transmitters equipped with a power monitor, it is essential to reduce the effects of load variations on the accuracy of the sampled output power. Accurate power monitor readings can be achieved by using either a high-directivity coupler, with greater than 15 dB directivity, such as shown in the schematic circuit diagram of FIG. 1, or a standard coupler cascaded with a circulator having one port terminated in a load, such as shown in the schematic circuit diagram of FIG. 2. FIG. 1 shows an amplifier 20 connected to a higher directivity coupler 22, which includes a primary transmission line 24 having in and out ports 26, 28, and a secondary transmission line 30 with a monitor port 32, coupling section 34 and load 36 connected to ground 38.
FIG. 2 shows a similar circuit, yet having the out port 28 connected to a circulator 40, and, in turn, connected to a load 42, connected to ground 45. The circulator 40 has an out port 44.
Cost and size quickly become key factors in choosing which system configuration to use. Different couplers have been used in prior art millimeter wave and other microwave coupling systems. For example, some waveguide circulators, such as manufactured by Flann Microwave, can be used at millimeter wave frequencies and integrated into Multipoint Video Distribution System/Local Multipoint Distribution Service (MVDS/LMDS) base stations or similar radio stations. One transmitter can feed a number of antenna arrays in point-to-multipoint transmitter systems. A microstrip circulator with the required ferrite puck mounted on top, such as produced by Renaissance Electronics Corporation, has also been used. A single junction microstrip circulator can include a stack-up of different parts, including a ground plane that could be metallized on the ferrite, and a ferrite disk with a conductive metal circuit having arms at 120 degrees relative to each other or at other angles as chosen by those skilled in the art. A spacer could be used to keep microwave fields out of the magnet and also supply a DC magnetic field. The phase shift between ports in the circulation direction could be 120 degrees using 120 degree spaced arms, while a phase shift in the opposite direction could be 60 degrees. For example, energy could be transmitted from port 1 to port 2 and shifted 120 degrees while energy from port 1 to port 3 could be shifted 60 degrees. Energy from ports 2-3 could be shifted 60 degrees. As another example, energy going either direction could be in phase at port 2 and adds together, while energy at port 3 is out of phase and cancels because no energy is transmitted to the port. Adding a termination at port 3 could convert the circulator to an isolator.
A high-directivity coupler is preferred over a standard coupler cascaded with a circulator because of its lower material cost, decreased assembly cost, smaller size, reduced complexity and temperature stability. Any directional coupler design is directed to how much directivity can be achieved from a given coupler. Directivity is therefore a qualitative benchmark by which couplers are compared.
High-directivity couplers can be fabricated in several technologies including waveguide, stripline or microstrip. Once again, however, cost and size are key factors. One type of standard high-directivity waveguide coupler, such as manufactured by Flann Microwave, is a three port design, and can include a low reflection termination built into the fourth arm. Standard coupling valves can be between about 10 and 20 dB.
A reduced-length high-directivity waveguide coupler, such as manufactured by Advanced Technical Materials, Inc. of Patchoque, N.Y., can be formed as a short length, high-directivity device, allowing a short insertion length and high-directivity. It can replace a cross-guide coupler where directivity is marginal and a short length is required. A typical, nominal coupling variation is about +/−0.75 dB, and a flatness is achieved of about +/−0.75 dB by using carefully controlled machining patterns. It can include a coupling of 30, 40 and 50 dB, and a frequency sensitivity and coupling accuracy of +/−0.5 dB. The directivity is about 25 MIN and 30 typical. The Voltage Standing Wave Ratio (VSWR) for the primary arm is about 1.05 and that for the secondary arm is about 1.25. These types of waveguide components are not preferred in some applications because of their high cost and large size.
Many Radio Frequency (RF) boards are designed in a microstrip (M/S) environment to facilitate the use of Monolithic Microwave Integrated Circuits (MMICs), any available Computer Aided Design (CAD) simulator models, and conventional test equipment. To integrate a stripline coupler into a microstrip environment requires an additional dielectric layer, an extra ground plane, and typically additional vias. Stripline couplers typically have not been preferred because of their increased complexity, substantial assembly time in manufacture, added material costs compared to other commercially available couplers, and increased labor costs associated with their manufacture and assembly.
Microstrip coupler designs have typically been more popular in use by circuit designers. High-directivity couplers that are compact, however, are difficult to design in a microstrip environment. A traditional microstrip coupler approach, for example, shown in the schematic circuit diagram of FIG. 3 and the fragmentary plan view of FIG. 4, has a quarter-wave coupling section with one port terminated into a load. FIG. 3 shows a linear, main transmission line 50 having in and out ports 52, 54, and secondary transmission line 56 that is U-shaped and includes a coupled port 58 on one leg and load 60 on the other leg and connected to ground 61. The coupling section 62 is shown by the two transmission line sections that are adjacent and parallel to each other. FIG. 4 shows a plan view of a microstrip example with elements in this microstrip example similar to those shown in the schematic circuit diagram of FIG. 3 having the same reference numerals. Unfortunately, this approach has often yielded relatively large couplers with poor directivity.
Some attempts to develop high-directivity microstrip couplers have been published, for example, in D. Brady, “The Design, Fabrication and Measurement of Microstrip Filter and Coupler Circuits,” High Frequency Electronics, July, 2002 volume 1, number 1; and M. Morgan, S. Weinreb, “Octave-Bandwidth High-Directivity Microstrip Co-Directional Couplers,” IEEE International Microwave Symposium, June 2003.
For example, a Schiffman, reduced-size directional microstrip coupler is shown in FIG. 5 at 70 and is reported to have some improvement in directivity. This coupler 70 includes a saw-tooth inner section 72 located between the main transmission line 74 and secondary transmission line 76. This coupler, however, is large and requires fine geometries. A backward wave microstrip coupler is shown in FIG. 6 at 80. It includes curved and adjacent transmission lines 82, 84. This coupler is also reported to have some improvement in directivity, but it is also extremely large.